Improves performance and includes a number of bug fixes. See
https://github.com/showdownjs/showdown/blob/master/CHANGELOG.md for
changes.

Several tests required small changes because the rendering of
`para1\n\npara2` changed from:

  `<p>para1</p>\n\n<p>para2</p>`

To the visually equivalent:

  `<p>para1</p>\n<p>para2</p>`